Video signal processing circuitry such as field/frame comb filters, field/frame recursive filters or progressive scan generators for example, have been designed which significantly enhance the quality of images reproduced from video signals. These systems perform very well for images which contain no motion (either image object motion or camera panning). Conversely, when image motion does occur, such systems tend to introduce undesirable artifacts. Consequently, these memory based processing systems are designed to be motion adaptive, i.e. the systems are either altered or switched out of the signal processing path when image motion occurs.
In order to alter motion adaptive systems during occurrences of image motion it is necessary to detect such occurrences. Typical motion detectors known in the art of video signal processing compare corresponding video signals from successive field or frame intervals. The assumption is made that if the interfield/frame video signals differ by more than a predetermined value motion has occurred.
Because video signals emanate from varying sources, e.g. different broadcast stations, VCR's, they tend to have varying signal-to-noise ratios (SNR's). Differences in SNR's complicates designing motion detectors to discriminate between image motion and image noise. One approach to discriminating between noise and motion is to average a number of signal differences corresponding to pixels surrounding the picture point being examined. Noise being uncorrelated, will tend to cancel. Signal differences corresponding to image object motion for localized pixels tend to exhibit a degree of correlation and thus add constructively.
The signal differences that are averaged are selected to correspond to pixels that are symmetrically disposed horizontally, vertically or both about the pixel being examined. Some systems weight the signal differences being averaged so that the signal averages exhibit a low-pass response.
Motion detectors which examine field/frame differences to determine the occurrence of motion are designed to operate on component rather than composite video signals. The reason for this is that chrominance components of composite video signals do not have similar phase relationships from frame-to-frame. Inherent frame-to-frame chrominance differences will produce motion signals even for still images. In order to preclude false motion detection when successive frames of composite video signal are compared, the composite video signal is typically limited to the low frequency luminance spectrum before comparison.
Examining only the low frequency spectrum of interframe composite video signal differences for motion, tends not to be satisfactory. In this instance motion of fine image detail cannot be detected. As an example, movement of a person's hair will not be detected in the reproduced image regardless of the speed of motion. Failure to detect motion of fine detail will generally tend to result in a blurred image.
Motion detectors which do operate on composite video (a) separate the luminance and chrominance components, (b) align the phase of the corresponding frame-to-frame chrominance components, (c) recombine the phase altered chrominance component with the luminance component and (d) take the interframe difference of the composite and chrominance phase altered composite video signals. This approach to detecting motion in composite video signals is not satisfactory because luminance and chrominance components cannot be completely separated in the chrominance spectrum of the composite video signal. The unseparated or residual luminance component in the separated chrominance signal undergoes an alteration during the chrominance phase alignment process. When this altered high frequency luminance is recombined and the interframe signal differences are taken, signal differences may occur in the absence of motion resulting in false motion detection.
An object of the present invention is to detect motion in the full spectrum of composite video signal with a minimum of false detection signals due to noise and cross components.